This invention relates to a liquid applying apparatus for applying liquid over a surface of a substrate utilizing centrifugal force.
There has been known a liquid applying apparatus which is used in the step of coating photosensitive material on a substrate in semiconductor device production. Such liquid applying apparatus to be used in the production of semiconductor device is required to uniformly apply a liquid containing photosensitive material onto a whole surface of a substrate such as wafer to form a very thin film over the whole surface of the substrate without any irregularity.
The conventional liquid applying apparatus includes a rotary member having a disk table on which a substrate to be applied with liquid is placed. The rotary member is rotated at a very high speed after a substrate is placed on the disk table and a drop of applying liquid is placed at a center of the substrate. The applying liquid drop spreads over a surface of the substrate owing to the centrifugal force of the high speed rotation of the rotary member, thereby forming a uniform thin film of applying liquid over the whole surface of the substrate.
The conventional liquid applying apparatus is provided with a cover member for defining a closed space above the disk table. The closed space above the disk table is necessary to form a uniform thin film on the surface of the substrate. In the case that the space above the disk table is not closed or opened to the outside, the air near the disk table is disturbed due to the high speed rotation of the disk table mounted on the rotary member. The disturbed air influences the centrifugal spreading of applying liquid and consequently causes irregularities in a liquid film formed on the substrate surface. The cover member is capped on the disk table to define a closed space to prevent the air disturbance due to the flow-in of air from the outside.
More specifically, the disk table has an annular vertical wall on a periphery of the disk table to surround the substrate placed in the disk table. The cover member is placed on a top surface of the peripheral vertical wall to define the closed space. The cover member is rotatably supported by a holder member. The cover member can rotate integrally with the disk table when the rotary member is rotated at the high speed. The holder member is mechanically connected with an elevating mechanism. The cover member is moved up and down by the elevating mechanism to open and close the space above the disk table.
In the conventional applying apparatus, to ensure the integral rotation of the disk table and the cover member, a standing pin is provided at a specified position of the periphery of the disk table while a fork portion is formed at a specified position of the periphery of the cover member. When the cover member is mounted on the disk table, a recess of the fork portion of the cover member is engaged with the standing pin on the top surface of the disk table.
However, the cover member is rotatably supported by the holder member. Accordingly, when the cover member is moved up and separated from the disk table, there is a likelihood that the cover member inadvertently rotate. This will make it cumbersome or difficult to engage the fork portion with the standing pin when mounting the cover member on the disk table. Further, if the rotary member is rotated at the high speed with the state where the cover member is not accurately set on the disk table, that is, the state where the fork portion is not engaged with the standing pin, the following problems will be likely to occur: Since the rotating force of the rotary member is not directly transmitted to the cover member, the disk table and the cover member hit against each other to generate vibration which will crack off dust powder from the cover member and the disk table. The cracked off dust powder will escape in the air around the apparatus or stray in the closed space to contaminate an applying liquid film being formed.
Also, to ensure the sealing of the close space above the disk table, a sealing ring or O-ring made of rubber is fitted in an annular groove formed in the top surface of the peripheral vertical wall of the disk table. It will be seen that to ensure airtightness of the closed space, the O-ring is required to partly protrude from the annular groove to enable contact with an underside surface of the cover member when the cover member is mounted on the disk table. Also, it will be seen that there is the necessity of preventing the O-ring from slipping from the groove formed in the disk table.
For these reasons, in the conventional liquid applying apparatus, the annular groove for retaining an O-ring is shaped into a trapezoid form in a radial section, that is, its top width is narrower than its bottom width. In other words, the opening width of the annular groove is smaller than the diameter of the O-ring.
However, since the opening of the annular groove is narrower than the diameter of the O-ring, the operation of inserting the O-ring in the groove is very troublesome. Also, it has been impossible to set the O-ring in the groove in the perfect state that the top line of the O-ring is on the same level along the entire circumference. Further, such forcible inserting of the O-ring has often caused polarizations in the elasticity of the O-ring along the circumference thereof.
Accordingly, in the actual setting of O-ring, the top line of the O-ring is not on the same level, but on different levels along the circumference, in other words, the level of the protruding portion of the O-ring is different along the circumference of the O-ring. The top line level difference of the O-ring causes gaps between the underside of the cover member and the O-ring, consequently breaking up the closed space above the disk table. As mentioned above, imperfect closed space above the disk table brings about the air disturbance, consequently making it impossible to form a uniform thin film on the substrate.